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Dew Strap Placement - Pro's and Con's?

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#1 junomike

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Posted 12 October 2013 - 02:32 PM

My last few outings have warranted the necessity for a Dew heater on my APM 130/780. I've been placing a DN008 (7") strap right at the back of the Dew Shield (just in front of the sloped slider). This has worked great as Dew never formed for the entire evening (6hrs). A few other scopes (an SCT and Apo) without Dew Straps were finished within the first hour or so.

Although the Dew Strap works fine when placed on the Dew Shield, I'm curious as I also have a Dew Strap that fits the smaller OTA housing directly behind the Dew Shield.
Reading previous posts and comments makes me wonder which way is more effective?

My current Dew Strap position is closer to the lens cell, but has to travel through the Kruppax Dew Shield which never makes direct contact with the lens cell Itself. My guess is that the minimal heat transferred to the inside of the Dew Shield is sufficient to keep the Dew away.

Placing the shorter Dew Strap directly behind the Dew Shield is more commonly recommended, but I'm wondering If this is time effective? The heat would have to travel from the OTA to the outer lens, by transferring heat through numerous glass surfaces as well as air spaces. This smaller Dew Strap draws less current to boot so It seems like It would be even less effective? Yet that's what most manufacturers recommend?

I haven't tried the shorter Dew Strap and I'm wondering If I should even bother? The larger Dew Strap draws more current which is good for effectiveness, but bad for the Battery. The shorter Dew Strap draws less, but will It work as well?

Mike

#2 orion69

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Posted 12 October 2013 - 02:53 PM

My current Dew Strap position is closer to the lens cell, but has to travel through the Kruppax Dew Shield which never makes direct contact with the lens cell Itself. My guess is that the minimal heat transferred to the inside of the Dew Shield is sufficient to keep the Dew away.

Correct.

Placing the shorter Dew Strap directly behind the Dew Shield is more commonly recommended, but I'm wondering If this is time effective?

Can you post a link where is that recommended? IMHO placing Dew straps between lenses could lead to deteriorating image quality because it could compromise scope's thermal equilibrium.

#3 Mike Lynch

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Posted 12 October 2013 - 03:15 PM

One short discussion on CN here:

http://www.cloudynig...3587309/Main...

#4 junomike

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Posted 12 October 2013 - 03:22 PM

Can you post a link where is that recommended? IMHO placing Dew straps between lenses could lead to deteriorating image quality because it could compromise scope's thermal equilibrium.



Knez, All the diagrams I've seen have an SCT as an example and the Dew Strap is placed behind the Corrector.
Also, the Manufacturers measurements and specs insinuate that the OTA and not Dew Shield is the proper placement.
i.e. in the Specification Table (D) they state that a DN006 is "best for" a 5" Scope. That would barely fit my APM 130/780 OTA let alone the Dew Shield.
Also in "B. Selecting a Dew-Not Dew Heater" They state "measure the circumference of your telescope tube (then X by 3.14).

You have me confused as placing the Dew Strap behind the Dew Shield isn't necessarily between the lenses?

Mike

#5 junomike

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Posted 12 October 2013 - 03:25 PM

One short discussion on CN here:

http://www.cloudynig...3587309/Main...


Mike, That was one of the confusing CN posts I read that initiated this post!

#6 orion69

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Posted 12 October 2013 - 09:39 PM

Knez, All the diagrams I've seen have an SCT as an example and the Dew Strap is placed behind the Corrector.
Also, the Manufacturers measurements and specs insinuate that the OTA and not Dew Shield is the proper placement.
i.e. in the Specification Table (D) they state that a DN006 is "best for" a 5" Scope. That would barely fit my APM 130/780 OTA let alone the Dew Shield.
Also in "B. Selecting a Dew-Not Dew Heater" They state "measure the circumference of your telescope tube (then X by 3.14).

You have me confused as placing the Dew Strap behind the Dew Shield isn't necessarily between the lenses?


Sorry, I was talking about refractor, no experience with SCT.

#7 GlennLeDrew

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Posted 12 October 2013 - 10:27 PM

Dew heaters work both via conduction and radiation. The heater in contact will transmit heat through the tube and/or cell into the Optic. At the same time, warmed tubing both behind and ahead of the optic will radiate into the optic. Radiation probably becomes more important the larger the diameter of the optic.

From experience, I've found that it differs little if the heater is behind, at or ahead of the optic. Indeed, it's not even really necessary for conduction to occur. A thermally isolated dew cap, when warmed, will do the job purely via radiative transfer.

#8 Skunky

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Posted 12 October 2013 - 10:31 PM

 

#9 GlennLeDrew

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Posted 13 October 2013 - 02:18 AM

Warming the air so that it then warms the lens is not the mechanism by which dew heaters work. For one thing it's not very efficient, for the low density of air makes it a poor conductor of heat. To be able to work like this, the air would have to be warmed enough to create harmful thermals.

The only way to get 'thermal runaway' is if so much current is applied that the resistive element gets sufficiently hot to cause the resistance to lower to the point where yet more current is passed, this running away until either destruction or a current limit is reached. Dew heaters cannot (or should not) ever get to this state.

#10 Mike Clemens

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Posted 13 October 2013 - 03:37 PM

For 10 years I've placed the strap at the back of the dewshield (on the dewshield), including on several TMB kruppax tubes. I've had no dew, and I've had several nights , sometimes straight in a row of 1.x" FWHM imaging, so I know it wasn't wrecking my seeing. This M33 was shot with a heater running on the dewshield... and I know this isn't having atmospheric problems:
http://www.pbase.com...162157/original
Have also had steady high resolution visual obs with the dewshield there, on many nights. Including seeing Deimos in a Kruppax tubed 175 with the heater as mentioned.

#11 cavefrog

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Posted 14 October 2013 - 02:21 AM

IMHO placing Dew straps between lenses could lead to deteriorating image quality


I have asked about this statement and what it means before and did not receive an answer.
"between lenses" what?.... does this mean between objective lens and eyepiece lens?

Theo

#12 orion69

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Posted 14 October 2013 - 03:21 AM

IMHO placing Dew straps between lenses could lead to deteriorating image quality


I have asked about this statement and what it means before and did not receive an answer.
"between lenses" what?.... does this mean between objective lens and eyepiece lens?

Theo


I'm talking about refractors. Refractors are doublets or triplets. Doublets have two lenses, front and rear, triplets have one more in the middle. If you put dew straps anywhere between front and rear lens then it's .... between lenses.

If you put dew straps on dew shield chances are that it is not between lenses like it's shown on this picture:



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#13 junomike

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Posted 14 October 2013 - 06:42 AM

If you put dew straps anywhere between front and rear lens then it's .... between lenses.


Actually, I did place the Dew strap right AT the lens cell (so in between my Triplet lenses). It seemed to work fine as the Dew shield doesn't actually touch the lens cell.
I'll try moving the Dew strap further forward next time.

Mike

#14 GlennLeDrew

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Posted 14 October 2013 - 05:08 PM

Wrapping the heater around the cell, or 'between the lenses', as it were, has no adverse effect. Again, around the dew cap, around the cell, or around the tube just behind the objective/corrector, is all much the same thing.

The amount of heat (nominally) being applied is merely compensating for that lost to the cold heat sink of the clear sky, keeping the optic temperature not much different from the ambient air temperature. It's not like you're pumping such a flux of heat into the scope that thermals or optic warping can result.

#15 orion69

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Posted 14 October 2013 - 05:18 PM

Wrapping the heater around the cell, or 'between the lenses', as it were, has no adverse effect. Again, around the dew cap, around the cell, or around the tube just behind the objective/corrector, is all much the same thing.

The amount of heat (nominally) being applied is merely compensating for that lost to the cold heat sink of the clear sky, keeping the optic temperature not much different from the ambient air temperature. It's not like you're pumping such a flux of heat into the scope that thermals or optic warping can result.


So, if you put dew straps near rear lens (towards focuser) in refractor that is over 1m or even 1.3m long, do you really think you can prevent dew on front cell?
Maybe it would work but for me it seams that best place is on dew shield near front lens since it is after all more or less closed system.






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